AC not turning on? Here is how to find the cause

First, which problem do you have?

An AC that "will not turn on" breaks three different ways, and knowing which one you have sends you straight to the right checks:

• Totally dead. Blank thermostat, nothing running inside or out. This is almost always power: thermostat batteries, a breaker, a power switch, or the drain safety switch. Start at the top of the list below.
• Indoor blower runs, outdoor unit dead. You feel air from the vents, but it is not cold and the unit outside is silent. Skip down to the outdoor disconnect, then the capacitor and contactor sections.
• Runs but blows warm. The system starts and moves air, it just is not cooling. That is a different problem with its own causes, covered on the AC blowing warm air page.
• Starts but will not stay on. It kicks on, runs a few minutes, shuts off, then restarts. That is short cycling, covered on the AC short cycling page.

Check the thermostat

The thermostat tells the AC when to run, so start there. Confirm three things:

• Set to COOL, not OFF or HEAT.
• Target temperature is below the room temperature. If the room reads 74 and you set 76, the AC has no reason to run. Drop it five degrees and listen.
• Batteries are good. A blank or dim screen on a battery model is a dead giveaway. Fresh batteries cost a couple of dollars and fix a surprising number of no-cool calls.

If the screen is completely blank and new batteries do not wake it, the thermostat itself or its wiring may be the issue, which the thermostat not working page covers. Give the AC a minute to start after you change a setting, since many systems wait a few minutes between cycles to protect the compressor.

Check the breaker, once

An air conditioner pulls a lot of power, and a storm or a power surge can trip its breaker. Open your electrical panel and look for the AC breaker, often a double-width one. A tripped breaker frequently sits in the middle, not fully on or off. Flip it firmly all the way OFF, then back ON.

Reset it only once. If the breaker trips again the moment you turn it back on, stop. A breaker that will not hold is protecting you from a real electrical fault, and flipping it repeatedly is a fire risk. That points to a deeper problem, covered on the AC tripping the breaker page, and it is a service call.

Check both power switches

Your AC has two shutoff switches, and either one being off leaves the unit dead. Both are common accidental trips:

• The indoor switch looks like an ordinary light switch and sits on or near the indoor air handler or furnace, often in a basement, closet, or attic. It gets flipped off by mistake all the time. Find it and switch it on.
• The outdoor disconnect is a gray box mounted on the wall right next to the outdoor unit. It has a pull-out block or a switch. Make sure the block is pushed all the way in and the switch is on. Someone may have pulled it during yard work, or it may have worked loose.

If the outdoor disconnect is seated and on but the unit is still dead, the box may hold cartridge fuses that blew. Replacing those is a small job for a tech, but a fuse that blows again means a real fault, so do not keep swapping fuses.

Check the air filter

A filter packed with dust starves the system of airflow. On some units that trips a safety and shuts things down, and on others it freezes the indoor coil into a block of ice that stops the AC entirely. Pull the filter and hold it up to a light. If you cannot see light through it, replace it with the same size, arrow pointing toward the unit.

If you find ice on the coil or the copper lines, shut the AC off and let it thaw fully before running it again, and see the AC freezing up page for why it happened. Our guide on how often to change a filter covers the schedule so it does not happen again.

Check the drain safety switch, the sneaky one

This is the cause almost nobody thinks of. Your AC pulls humidity out of the air, and that water drips into a pan and out through a drain line. When the drain line clogs with algae and gunk, the water backs up, and a small float switch rises and shuts the entire system off on purpose to keep the overflow from flooding your ceiling or floor. The AC looks dead, but the real problem is standing water.

With the thermostat off, find the drain pan under the indoor unit and look inside with a flashlight. If there is standing water, stop and clear the drain before you run the system again, because restarting it risks water damage. You can often pull the clog by holding a wet/dry vacuum to the end of the outdoor drain line where it exits the house, which sucks the blockage out. Once the water drops, the float switch resets itself. Never tape or jam the float switch to bypass it, since the clog is the problem, not the switch. If the line will not clear, a tech can do it, usually for $75 to $250.

Try a full reset

If everything above checks out and the AC still will not start, give it a full reset. Turn the thermostat off, switch the AC breaker off, wait five to thirty minutes, then turn the breaker back on and the thermostat to cool. This clears the control board and lets the system start fresh, and it often clears a unit that locked itself out after a power blip.

A reset that brings the AC back to life is fine. A reset you have to do again every day is masking a real fault, so if you find yourself resetting it over and over, stop and have it looked at rather than nursing it along.

Outdoor unit hums but the fan will not spin: likely the capacitor

If the indoor blower runs and the outdoor unit hums or buzzes but the fan blade does not turn, that is the classic sign of a failed capacitor. The capacitor is the part that gives the fan and compressor the jolt they need to start spinning. When it dies, the motors try to start, cannot, and just hum. Shut the unit off rather than leaving it humming, because running a motor that cannot start can overheat and ruin it.

A capacitor is the single most common AC failure and one of the cheaper fixes. It is a tech job, not a DIY one, because the part holds a charge that can shock you even with the power off. The cost is in the cost list below, and our AC capacitor replacement guide covers the signs, the cost breakdown, and why this is the one repair to leave to a tech.

Outdoor unit silent with the blower running: the contactor

If the indoor side runs but the outdoor unit is completely silent, the contactor is a likely cause. The contactor is the switch that sends power to the outdoor unit when the thermostat calls for cooling. When its contacts wear out, get stuck, or burn, the outdoor unit never gets the signal to start, so it sits there dead while everything indoors seems fine. Replacing it is a quick tech job, priced below.

Repairs that need a tech, and what each one runs

If the free checks did not bring it back, you are into parts a technician replaces. These are installed prices, parts and labor together, so you know whether you are looking at a cheap fix or a big one before the truck shows up. Regional labor rates move these around.

If a tech says the compressor is the problem on an AC that is ten years or older, price a full replacement before you commit, because a $1,500 to $2,800 compressor on aging equipment is often money better put toward a new system. The central AC cost guide covers when repair stops making sense, and you can get a free quote from a local installer to compare against the repair bill.

When to stop and call

Some signs mean you stop troubleshooting and pick up the phone:

• The breaker trips again right after you reset it. That is a short or a failing part, not a nuisance trip.
• The outdoor unit hums but the fan will not spin. Shut it off so it does not burn out the motor, then call.
• There is standing water in the drain pan. Clear the drain before running the system, or have a tech do it.
• You smell burning or see scorch marks at the indoor or outdoor unit.
• You keep having to reset it to get any cooling at all.

None of those are worth the risk of a DIY save. Shut the AC off at the breaker and call a licensed tech. Most no-cool calls turn out to be a capacitor or a clogged drain, so the repair is often far cheaper than people fear.

Common questions about an AC that will not start

Why is my AC not turning on but the thermostat is on?

Usually a power problem between the thermostat and the unit. Check that the thermostat is set to cool with the temperature below the room, then the breaker, then both the indoor and outdoor power switches. If all of those are good and nothing happens, a blown low-voltage fuse or a bad control board can stop the unit even with a working thermostat, and that is a service call.

Why is my inside unit running but the outside unit will not turn on?

Start with the outdoor disconnect box on the wall by the condenser and make sure it is seated and on. If it is, the most likely causes are a failed capacitor (the unit hums but the fan will not spin) or a worn contactor (the unit is completely silent). Both are tech repairs, and both are usually inexpensive.

Is there a reset button on my AC?

Some outdoor units have a small red reset button, but most do not. The reliable way to reset a central AC is to turn the thermostat off, switch the breaker off for five to thirty minutes, then turn it back on. If it locks out again soon after, it is catching a real fault rather than glitching.

Why does my AC hum but not start?

A hum with no fan movement is the textbook sign of a failed capacitor, the part that gives the motors their starting jolt. It is common and relatively cheap to fix, but do not leave the unit humming, because a motor that cannot start can overheat. Shut it off and call.

Can a clogged drain really shut my whole AC off?

Yes. High-efficiency and most modern central systems have a float switch that cuts power to the whole AC when the condensate drain backs up, so it does not flood your home. It is a built-in safety, not a fault. Clear the drain and the system comes back on its own.